The present invention is directed to a switching amplifier of the type having a sequence of voltage sources which are switched in response to a low frequency input signal, and more particularly to a switching amplifier for generating a low-frequency modulated anode voltage for a high-frequency, high-power AM transmitter, which, e.g., has an output of approximately 600 kW.
German Offenlegungsschrift [patent application laid open without examination] No. 1,816,530 is directed to a switching amplifier in which an output voltage is controlled in response to an analogous input signal. In this power amplifier several voltage sources are connected in sequence such that a specific voltage source is allocated to each voltage range.
In order to obtain a highly linear output signal one must either use a substantial number of voltage sources, which are then merely switched ON or OFF, or a few voltage sources whose respective output voltages are analogously controlled in a certain range. Grid-controlled tubes and/or transistors can be used as controlling elements.
The disadvantage of such an arrangement is that it can only be used as an alternating voltage amplifier of little power for a frequency range of 40 Hz to 4 kHz. In addition, small-scale gradation of the output voltage in a cost-effective manner (e.g., in a binary manner or by means of pulse width modulation) is not possible, since the voltage sources are always sequentially switched ON or OFF corresponding to the magnitude of the control signal, which, disadvantageously, is of the same magnitude.
From the German Offenlegungsschrift No. 3,044,956 as well as the European patent application No. 66,904 A1, switching amplifiers which avoid the disadvantages mentioned above are known. However these amplifiers have the disadvantage of producing a capacitive load on the output voltage, which is caused by transformer coils associated with the voltage sources.